1. Field of the Invention
This invention relates to optical transmission systems. More particularly, the invention relates to an optical transmission system having a "sub-center type arrangement" in which signals are transmitted from a central source to a plurality of sub-centers through optical fibers and are then transmitted from each sub-center to a plurality of subscribers through other optical fibers.
2. Description of the Prior Art
A variety of applications of optical fiber cables have been proposed such as, for example, two-way CATV systems. FIG. 1 shows a conventional general arrangement in which optical fiber cables extend from a CATV center 10 to each of subscribers 12-1, 12-2, . . . , 12-n through a separate optical fiber. The arrangement illustrated in FIG. 1 may readily be employed when the number of subscribers is small. However, in an urban CATV system, where there may be more than several hundred thousand subscribers, the optical fiber distribution system of FIG. 1 becomes unwieldy due to the large number of optical fibers that must connect center 10 with subscribers 12-1 through 12-n.
It has been proposed in the past that the difficulties inherent with the arrangement illustrated in FIG. 1 may be eliminated by so-called "sub-center type switched-star network" as illustrated in FIG. 2 in which the overall service area is divided into a number of sub-areas which each includes a sub-center. Signals from a CATV center 10 are transmitted along the transmission lines A to each of sub-centers 14-1 to 14-X and each of the sub-centers retransmits the signals to a number of subscribers over lines B. Thus, as shown in FIG. 2, CATV center 10 is connected to sub-centers 14-1, 14-2 and 14-3 and sub-center 14-4 is connected to sub-center 14-3 over links A. Sub-center 14-1 is connected to subscribers 16-1 to 16-n. Sub-center 14-2 is connected to subscribers 18-1 to 18-n, sub-center 14-3 is connected to subscribers 20-1 to 20-n, and sub-center 14-4 is connected to subscribers 22-1 to 22-n all over links B.
In the CATV system shown in FIG. 2, links A and links B are used for picture signal transmission. However, as a result of the arrangement of the system, links A and links B should optimally have different performance standards and carry picture signals over different distances. Accordingly, in an optical system, transmission over links A should be different from the transmission over links B. Table 1, on this page of this application, summarizes the inherent differences between links A and links B in the system illustrated in FIG. 2.
TABLE 1 ______________________________________ CONDITIONS REQUIRED FOR LINKS A AND B ITEM REQUIREMENTS LINK A LINK B ______________________________________ 1 No. of Transmis- 30-50 Channels At Least 2 sion Channels Per Sub-center Channels Per Subscriber 2 Number of Optical 30-50 Cores-One One Core Per Fiber Cores For Each Channel Subscriber Is Acceptible Desirable To Minimize Total No. 3 Transmission 2 to 10 Km Less Than Distance or More 2 Km ______________________________________
It is different conditions for transmission over links A and links B as summarized in Table 1 that warrant the difference in transmission systems employed over links A as compared to links B. However, employment of different transmission systems for links A and B makes a CATV system intricate and increases operating costs.